The present invention relates, in general, to plastic molds or castings for making investment molds and, more particularly, to a new and useful round ringless mold and spoked sprue for making investment molds for precision casting of dental prostheses.
In the dental field, it is common to use lost wax methods for producing dental castings Lost wax methods utilize investment solution which is at least partly cured in a vessel in order to produce a dental casting.
In many known casting devices, a metal or plastic ring is force fit into a groove in a base to form the mold or vessel for investing the investment solution.
The use of polyvinyl chloride (PVC) or other elastomer, in the construction of both the base and ring is desirable, among other reasons, because such materials can accommodate expansion of the investment casting as it cures and such materials are more readily cleaned after the hardened investment is removed.
Molds and sprues in this field are illustrated in U.S. Pat. Nos. 4,573,921; Des. Pat. No. 329,900; 5,406,999 and 5,469,908.
Various patents in this field disclose various sprue plus mold structures. See for example U.S. Pat. Nos. 1,458,835; 1,939,479; 2,274,186; 3,648,760; 3,939,898 and 4,558,841.
U.S. Pat. No. 4,962,909 discloses a mold for dentistry which includes a lower funnel shaper for shaping the funnel portion of an investment, and an upper blind funnel for displacing a core portion of the investment from within a pattern of dental forms, to reduce distortion in the forms which would have been caused by an expansion of the investment as it cured.
U.S. Pat. No. 5,183,095 discloses a mold for precision casting made of thermoplastic material.
The advantage of using thermoplastic material for a mold in the field of dental casting has been described in Materials and Materials Science, No. 3 Report 303, March 1981, "The Equi-Spansion Technique for Castings Without Metal Rings" by Engelmann and Blechner. The advantages of maintaining uniform investment wall thickness around a wax form set by using an oval mold for making a precision casting has been disclosed in Trends & Techniques in the Contemporary Dental Laboratory, Volume 8, No. 3, "The Effects of Shape and Size of Investment Heating and Cooling Rates", by Berger and Benson.
A wax sprue for dental casting is also currently being marketed by American Diversified Dental Systems, known as The Spiroloop. This product comprises a central post or hub, meant to be attached to the base of a mold. Three spokes extend outwardly from the hub and are curved helically. A ring connects the outer ends of the spokes and forms a support for wax forms to be attached to the top of the sprue. The spokes and ring all have circular cross-sections. Accordingly, after the lost wax process is completed, the metal casting includes thick cylindrical and helical spoke segments and a thick ring with cylindrical cross-section. This represents the use of a large quantity of metal. The advantages of the spiral wound spokes are said to include a more quick and smooth flow of metal without the creation of restrictions that cause turbulence.
A need remains for an improved mold with sprue that reduces distortion of the wax forms in the investment casting, reduces the amount of wasted investment casting and reduces the amount of excess metal in the sprue pattern of the precision casting.